Digitizers and tablets can be incorporated as a coordinate input apparatus in processing units. For instance, the digitizer or tablet can be used alongside one or more display devices (e.g. CRT, LCD, or other display technology) in a touch enabled display assembly. Generally speaking, various systems for detecting an angle (direction) or a position of an object relative to the display area can be used, such as pressure sensitive resistance membrane systems, capacitance systems, electromagnetic induction systems, and the like. As another example, optical systems capable of detecting the angle or the position of the object can be used.
More particularly, touch screen input devices include resistive, surface capacitive, surface acoustic wave (SAW), infrared (IR), Frustrated Total Internal Reflection (FTIR), Projected capacitive, optical and bending wave. Often, the foregoing touch screen devices (aside from some optical and infrared technologies) require use of a touch enabled transparent cover layer that adds height to the display assembly.
Certain optical and infrared systems rely on detection of light traveling in optical paths that lie in one or more detection planes in an area (“touch area” herein) above the touched surface. For example, optical imaging for touch screens can use a combination of line-scan or area image cameras, digital signal processing, front or back illumination, and algorithms to determine a point or area of touch. Components used to emit and detect light in the detection plane(s) can be positioned along one or more edges of the touch screen area as part of a bezel surrounding the touch screen area. Optical touch technology often uses line-scanning or area cameras orientated along one or more edges of the touch surface to image the bezel and track the movement of any object close to the surface of the touch screen by detecting the interruption of an infrared light source.
In some systems, the light can be emitted across the surface of the touch screen by IR-LED emitters aligned along the optical axis of the camera to detect the existence or non existence of light reflected by a retro-reflective border. If an object is interrupting light in the detection plane, the object will cast a shadow in the retroreflected light. Based on the direction of the shadow as cast toward multiple detectors and the spatial arrangement of the detectors, the object's location in the detection area can be triangulated. As another example, light can be emitted across the touch area in a grid pattern, with the object's location determined based on where the grid is interrupted.
For instance, FIG. 1 is a cross-sectional view of an exemplary touch detection system 10 in which an optical detection and illumination system 12 and illuminated bezel 14 both extend above a touch surface 18. In this particular example, the optical detection system and illumination system are combined into a single unit as is known in the art. The plane of touch surface 18 in this example corresponds to the top of display screen 16 or a protective layer positioned above the display screen. Light is emitted from illumination source 20 and is directed along an outgoing optical path 20. The light is then retroreflected along return optical path 24, passing into optics 28 (a lens in this example) and detector 26. In this example, the profile of the touch detection system is 3.2 mm.
As is known to those of skill in the art, the triangulation principle can be used to calculate the position at which an object impinges on a detection plane via measurements from two or more detection systems. FIG. 2 provides a top view of an exemplary touch detection system 29 which can identify coordinates within a detection plane 31. In this example, two optical detection and illumination systems 30 are provided, with respective exemplary paths 34 and 36 showing the route of light emitted from and returned to the illumination/detection systems 30 via reflective components positioned along edges 32. For example, retroreflective components can be positioned along edges 32 covered by or included in a bezel.
When an object interrupts the beams as represented at 33, the location of the interruption can be triangulated based on the change in optical paths across detection plane 31. For example, an object may cast shadows which are detected by illumination/detection systems 30, with location 33 triangulated from the direction of the shadows and the known spatial relationship between illumination/detection systems 30.
FIG. 3 shows a side view of another exemplary touch detection system 40. In this example, illuminated stylus 42 is used to intersect detection plane 44. Light traveling in detection plane 44 may be collected via optics 46 and directed via reflector 48 to detector 50. In this example, detector 50 is interfaced with detector electronics 52 mounted above the touch surface. For example, detector 50, electronics 52, lenses 46, and reflector 58 may all be built into a bezel surrounding screen 58.
The relative complexity of the optical components used to emit and detect light can lead to a profile height of the bezel that is not suitable for all applications. For example, the bezel height may be too large for use in a handheld computing device, such as a mobile phone, or personal digital assistant (PDA).
RPO Pty Ltd of Australia, attempts to provide a low profile by having the IR emitters and receivers optically connected by wave guides. In the example of FIG. 4, touch-enabled display 60 comprises an LCD display 62 surrounded by a plurality of transmit side waveguides 64 and receive side waveguides 66. Transmit side waveguides provide an optical path from light source 68, while receive side waveguides provide an optical path to detection electronics (ASIC) 70. Light from transmit side waveguides 64 forms a grid pattern across display 62 which can be detected by electronics 70. An object's location on the detection plane can be determined based on interruptions or other disruptions in the expected grid pattern. For example, source 68 may emit infrared or other light, and interruption of the grid may result in a shadow that diminishes the light received at one or more receive side waveguides 66.
Although waveguides 64 and 66 allow for source 68 and electronics 70 to be positioned below the screen surface, the waveguides add cost and complexity to the touch-enabled display. For example, the waveguides may be fragile and require careful handling. As another example, each of several waveguides must be connected to the touchscreen at one end and to electronics 70 at the other end. In short, use of the waveguides can complicate assembly and repair and may lead to a less durable product